Why Didn't Leia Show More Sadness?

In A New Hope, after Obi-Wan Kenobi becomes one with the Force, our heroes escape the Death Star on the Millennium Falcon. While Luke is mourning the death of his teacher, Leia comforts him in his grief. Some fans consider this rather odd, considering the princess' planet was just obliterated. This begs the question: why wasn't Leia sad? As it so happens, I have a theory on that. Join me and we'll discuss!